Track repr(packed) and track unsafe references of fields to packed structs.

[Nashenas88]

The comment chain here #4587 (comment) requests tracking the unsafe behavior of taking references to fields of packed structs. Given that PR was starting to get too large, so this one was created to track related work.

[Nashenas88]

@matklad I forgot to update PR once it was split. This work is being done in #4743